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, large assemblage of loosely related, wormlike organisms of extremely varied structure and habits. Formerly considered an animal phylum, these organisms are now more commonly referred to as pseudocoelomates and are divided in to a number of different phyla,
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A phylum of minute metazoan animals (formerly placed in the aschelminth group) numbering 500 described species worldwide. Some 300 species have been reported from the marine habitat, with new ones being described every year.
Gatrotrichs comprise two orders, the Macrodasyida and the Chaetonotida. The term Gastrotricha refers to the ventral locomotor cilia by which the animals glide gracefully over the substratum or through its interstices; unlike many other ciliated animals, they cannot move in reverse. Gastrotrichs have a complete digestive tract, with a sucking pharynx, a simple intestine with a wall only a single cell thick, and an anus. They appear to be selective feeders on bacteria, very small protozoa, and yeasts. Most have protonephridia, accounting in part for their broad salinity tolerances.
Gastrotrichs appear to be regionally cosmopolitan, with 20–30% having broad distributions within continents, and 10–15% between continents; endemism probably does not exceed 20%.
The phylum Gastrotricha is the most primitive in the aschelminth group of phyla. Gastrotrichs and nematodes probably share a common ancestor, which in turn was descended from a stock that included gnathostomulids and turbellarianoid animals. See Nemata
a class of lower worms of the phylum Nemathelminthes (roundworms). It was identified by the Russian biologist I. I. Mechnikov. There are approximately 200 species of these microscopic aquatic animals. The body (up to 1.5 mm long) is elongated and wormlike, with cilia on the ventral side. On the skin there are cuticular scales or bristles and special cement glands. The central nervous system consists of paired cerebral ganglia and two lateral nerve trunks. The sense organs are small pigmented eye spots, tactile bristles, and small olfactory pits on the body. The muscles are in the form of separate thin muscle bundles. The excretory organs are a pair of short coiled pronephridia ducts that open to the exterior on the sides of the body. Gastrotricha are hermaphroditic or have separate sexes. They usually have paired reproductive organs.